In recent years, the Global System for Mobile Communications (GSM) and other cellular networks have grown rapidly all over the world. An accordingly increasing number of subscribers has led to a corresponding increase of bandwidth in trunking networks of cellular systems such as base station subsystems (BSS) in the GSM.
FIGS. 5a and 5b schematically show examples of parts of trunking networks in mobile communication systems. In such trunking networks, a mobile services switching center MSC and an associated visitor location register VLR are linked to a base station controller BSC. The base station controller BSC is in turn linked to a base transceiver station BTS, which is coupled with mobile stations (not shown) at respective end users. The base station controller BSC and the base transceiver station BTS constitute a so called base station subsystem BSS, which is indicated by dashed lines in FIGS. 5a and 5b. 
In more detail, in FIG. 5a part of a conventional pulse code modulation (PCM) based trunking network is illustrated, in which the coupling links, i.e. the connections between network elements, and the switching in the base station controller BSC are all PCM based.
However, the integration of mobile stations such as mobile phones and data communication has recently given rise to the introduction of new data services like e.g. the Short Message Service (SMS) or the access to the Internet using a mobile station. Along therewith, an increasing demand for replacing these conventional narrow banded PCM networks with broadband communication systems can now be observed.
As a future data transfer technology for use in broadband communication systems, asynchronous transfer mode, in short also referred to as ATM, has been proposed.
The basic idea of asynchronous transfer mode is to transfer data in small data packets having a fixed size. These data packets are called cells. A flow of such cells represents a virtual channel connection VCC between respective end users. The virtual nature of the connection arises from the fact that cells are transferred only when there are data to be sent. In contrast to known connection-oriented systems, therefore, resources are not used when no useful payload is present.
Asynchronous transfer mode is also suitable for transferring packet data in e.g. GSM networks. In this respect, General Packet Radio Services (GPRS) have been proposed, in which data packets like e.g. IP traffic are transferred through GSM radio interface.
However, with a conventional PCM based base station subsystem as shown in FIG. 5a, such data packets must first be transformed into PCM format in the base transceiver station BTS and then retransformed into packet format before being sent to public packet data networks like e.g. the Internet. The same transformations must also be made for data packets traveling in the opposite direction, i.e. from the public data networks to the base transceiver station BTS. By use of asynchronous transfer mode procedures, this data transformation can be avoided, and it is moreover possible to directly route data packets in asynchronous transfer mode cells from the base station subsystem BSS to public asynchronous transfer mode based networks.
In order to use asynchronous transfer mode in the trunking network of a GSM system, at least the conventional PCM links between respective network elements must be replaced with ATM links. In the resulting arrangement, ordinary PCM based GSM network elements are then connected by ATM links, for which an example is shown in FIG. 5b. 
Such a technique is called Circuit Emulation Services (CES), which, however, is not yet an effective realization of ATM, since the base station controller BSC and, thus, the base station subsystem BSS do in this case not support ATM switching.
Therefore, it is an object of the present invention to provide a device which is capable of supporting ATM switching at base station subsystem level and, thus, implements an effective realization of asynchronous transfer mode in mobile communication networks.
In view of the future expansions expected in such mobile communication networks, the present invention shall also provide a device which can be manufactured, installed and maintained at low cost. Asynchronous transfer mode technology may also provide a big increase in capacity when compared with PCM technology. Thus, the number of physical lines in base station subsystems can be reduced.
According to the present invention, this object is accomplished by a broadband cellular network device, comprising a base station control unit adapted to control the distribution of asynchronous transfer mode cellular traffic consisting of asynchronous transfer mode cells, an asynchronous transfer mode controller connected to and being controlled by said base station control unit, and an asynchronous transfer mode switching means connected to and being controlled by said asynchronous transfer mode controller and adapted to switch asynchronous transfer mode cellular traffic.
With the above configuration, an asynchronous transfer mode based broadband cellular network device is implemented by combining the functions of a base station controller unit with an asynchronous transfer mode switch.
The device is capable of performing asynchronous transfer mode switching in a base station subsystem and moreover represents an inexpensive basis for an asynchronous transfer mode based base station controller for use in asynchronous transfer mode based cellular networks.
Further embodiments of the present invention are subject of the attached dependent claims.
Preferably, said base station unit provides either of a software, hardware or mixed software/hardware implementation of base station controller functions and comprises an asynchronous transfer mode controller instruction means.
The asynchronous transfer mode controller is arranged to provide an interface for converting commands issued by the base station controller unit into commands causing switching actions of the asynchronous transfer mode switching means.
Advantageously, the asynchronous transfer mode controller is adapted to employ asynchronous transfer mode based signalling and to provide control commands for controlling connecting hardware of the asynchronous transfer mode switching means.
According to a further development, the asynchronous transfer mode controller is arranged to comprise at least two functional layers, one of the functional layers being a cellular network related upper layer adapted to perform cellular network related functions, and one of the functional layers being an asynchronous transfer mode related lower layer adapted to perform asynchronous transfer mode switching means related functions.
Thereby, the lower functional layer of the asynchronous transfer mode controller is advantageously arranged to control the switching hardware of the asynchronous transfer mode switching means.
In addition, the asynchronous transfer mode controller may be adapted to be a General Switch Management Protocol controller. Then, the asynchronous transfer mode switching means is adapted to support said General Switch Management Protocol.